Can I blend 100LL with Mogas with ethanol?

15% and under should not be an issue for anyone. Some states in the US use up to 15% during the winter and many Rotax owners have used it for at least 2 decades. Fuel is mixed at the tank farms when the fuel trucks are loaded. In many states I know of people that always test their ethanoil content level. In Arizona where they say up to 10% it is usually only 6% - 7%.  So unless someone does an easy ethanol check on the fuel at their location then it would be hard to lable someplace that says up to 15% to actually be that. Believing on what the fuel pump says isn't the best indication of the true level of ethanol because it's usually lower.

Do we need to clean, service or replace the O2 sensor if using Decalin with 100LL

New to the 915is engine:  Where is it located and how difficult to do if it needs cleaning/service/replacement?

You never need to Clean, Service, or Replace the Oxygen Sensor.
The engine does NOT have one!

Bill is correct, no O2 sensor.  An O2 sensor would fail very fast with leaded fuels so none were used in production engines.  

I feel that some people are a bit off on some of the other parts of the thread, especially on ethanol.  The 10% max value from Rotax on what can be used is for several reasons.  it is true that engines were tested with higher levels however there are some serious concerns with the engine's emissions (pollution) when we exceed 10% .  higher than 10% leads to significantly higher levels of NOx emissions into the environment.  As a point of interest you will see emissions get better if you exceed 30% ethanol but that then creates other problems in that the fuel burn (Stoichiometric ratio) changes a great deal when you go that high.  Rejetting or in the case of the injected engines remapping of the fuel flow should be done to correct for the burn to be inline with such fuel.  Given that is not possible for the injected higher ethanol volumes may see detonation problems at high levels of ethanol.  

The question of blending 100LL with whatever fuel you would have locally, such as the 27% ethanol auto fuel in Brazil, is a valid one.  The short answer is sure, go ahead.  Fuels will blend find.  Ethanol however is hydroscopic and a 50/50 blend will cut the ethanol total volume down to ½ of the 27 you have.  The blend then would drop into about 13 or 14 by volume of ethanol, much better than the straight auto fuel there of 27.  The higher MON octane rating of the 100 LL will then increase the auto fuel MON rating also, so it should be a good blend to use.  

Let us all remember that the fuel companies in addition say cut the max storage time before use to less than 3 months from the time it was new.  If it was my engine I would not use it when it is older than say 30 days if possible.  At the very least if the fuel time is unknown top up with 100 LL Avgas to try and avoid detonation.  it will blend fine. 

Cheers